Atom-Based Electromagnetic Radiation Electric-Field Sensor

ABSTRACT

A method is presented for measuring the electric field of electromagnetic radiation using the spectroscopic responses of Rydberg atoms to the electromagnetic radiation field. The method entails implementing quantitative models of the Rydberg atom response to the electromagnetic radiation field to provide predetermined atomic properties or spectra for field amplitudes and or frequencies of interest, spectroscopically measuring the response (spectrum) of Rydberg atoms exposed to an unknown electromagnetic radiation field, and obtaining the electric field amplitude and/or frequency of the unknown electromagnetic radiation by using features extracted from the measured spectrum and comparing them to features in a predetermined spectrum among the set of predetermined spectra.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/175,805, filed on Jun. 15, 2016. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to a technique for measuring the electricfield (or frequency) of electromagnetic radiation using the response ofRydberg atoms.

BACKGROUND

Significant progress has been made in recent years towards establishingatomic measurement standards for field quantities. Rydberg atoms holdparticular appeal for applications in electrometry due to their largetransition electric dipole moments, which lead to a strong atomicresponse to electric (E) fields. Rydberg electromagnetically inducedtransparency (EIT) in atomic vapors has recently been demonstrated byapplicants as a practical approach to absolute measurements ofradio-frequency (RF) E fields over a broad frequency range (10 MHz to500 GHz) and dynamic range (˜100 mV/m to >1 kV/m) suitable for thedevelopment of calibration-free broadband RF sensors. The utility of theRydberg EIT technique in characterizing RF E fields has beendemonstrated in a number of applications. These include microwavepolarization measurements, millimeter-wave (mm-wave) sensing, andsubwavelength imaging. The approach has also been employed inroom-temperature studies of multiphoton transitions in Rydberg atoms, aswell as in measurements of static E fields for precise determinations ofquantum defects.

The Rydberg EIT measurement technique has been employed in measurementsof weak RF fields. In the weak-field regime, the atom-field interactionstrength is small compared to the Rydberg energy-level structure, andthe level shifts of the relevant coupled atom-field states are welldescribed using perturbation theory. By exploiting near-resonant andresonant dipole transitions between high-lying Rydberg levels, whichelicit a maximal atomic response, RF fields from as small asapproximately 100 mV/m to a few tens of V/m have been measured. Formeasurements of strong RF E fields, the atom-field interaction cannot bemodeled using perturbative methods and requires a non-perturbativemethod to accurately describe the response of the atomic system.Extending the atom-based measurement approach to a high-power regimecould enable, for example, subwavelength characterizations of antennasradiating high-power microwaves among other applications.

This section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A method is presented for measuring the electric field ofelectromagnetic radiation using the spectroscopic responses of Rydbergatoms. The method includes: providing predetermined atomic spectra foratoms of a known type; placing the atoms within the unknownelectromagnetic radiation field to be measured, where the atoms are in agaseous state and contained in a vacuum enclosure; propagating one ormore light beams through the atoms, where at least one light beam iscoupled to a Rydberg state; measuring an atomic spectrum using the oneor more light beams while the unknown electromagnetic radiation isinteracting with or has interacted with the atoms; analyzing themeasured atomic spectrum to extract spectral features; comparing thespectral features from the measured atomic spectrum to spectral featuresof the predetermined atomic spectra; matching the measured atomicspectrum to a given spectrum in the predetermined atomic spectra; andquantifying at least one of field strength or frequency of the unknownelectromagnetic radiation field using the given spectrum and thepredetermined atomic spectra.

Predetermined atomic spectra for the atoms are models of atomicresponses in presence of the electromagnetic radiation. In oneembodiment, the predetermined atomic spectra are calculated at a fixedfrequency for a range of electric field values. In another embodiment,the predetermined atomic spectra are calculated at a fixed electricfield for a range of frequencies. In some embodiments, the predeterminedatomic spectra are calculated using Floquet theory.

In some embodiments, the atomic spectrum can be measured usingelectromagnetically induced transparency. For example, a probing lightbeam is propagated through the atoms, where the probing light beam has afrequency resonant with transition of the atoms from a first quantumstate to a second quantum state; a coupling light beam is propagatedthrough the atoms simultaneously with the probing light beam, where thecoupling light beam is overlapped spatially with the probing light beam,frequency of the coupling light beam is scanned across a range in whichatoms transition from the second quantum state to a Rydberg state; andthe probing light beam passing though the atoms is detected using alight detector. In another example, a probing light beam is propagatedthrough the atoms, where frequency of the probing light beam is scannedacross a range in which atoms transition from a first quantum state to asecond quantum state; a coupling light beam is propagated through theatoms concurrently with the probing light beam, where the coupling lightbeam is overlapped spatially with the probing light beam, frequency ofthe coupling light beam is resonant with transition of the atoms fromthe second quantum state to a Rydberg state; and the probing light beampassing though the atoms is detected using a light detector.

In one aspect of this disclosure, spectral features extracted from themeasured atomic spectrum are further defined as the frequency differencebetween two split peak pairs in the measured atomic spectrum. Thesespectral features can be compared by overlaying the predetermined atomicspectra onto the measured atomic spectrum and shifting the predeterminedatomic spectra such that the predetermined atomic spectra aligns withthe measured atomic spectrum. In this case, field strength of theunknown electromagnetic radiation field can be quantified by determiningRabi frequency from a splitting of a Rydberg line in the measured atomicspectrum, calculating dipole moment of the relevant Rydberg transition,and computing magnitude of field strength of the unknown electromagneticradiation field from the Rabi frequency and the dipole moment.

In another aspect of this disclosure, the spectral features extractedfrom the measured atomic spectrum are further defined as one or more ofpeak heights, peak widths and relative peak positions in a Floquet map.These spectral features can be compared by overlaying the predeterminedatomic spectra onto the measured atomic spectrum, shifting thepredetermined atomic spectra in relation to the measured atomic spectrumso that the spectral features are in agreement, thereby yielding thefield strength or frequency of the unknown electromagnetic field.

A system is also presented for measuring the electric field ofelectromagnetic radiation using spectroscopic responses of Rydbergatoms. The system includes: a vapor cell containing atoms of a knowntype; a source of electromagnetic radiation arranged to emitelectromagnetic radiation towards the vapor cell; a probing light sourceconfigured to propagate a probing light beam through the vapor cell,where frequency of the probing light beam is scanned across a range inwhich the atoms transition from a first quantum state to a secondquantum state; a coupling light source configured to propagate acoupling light beam through the vapor cell concurrently with the probinglight beam, where the coupling light beam is counterpropagating to andoverlapped spatially with the probing light beam, and frequency of thecoupling light beam is resonant with transition of the atoms from thesecond quantum state to a Rydberg state; a light detector configured toreceive the probing light beam after passing through the vapor cell; adata store that stores predetermined atomic spectra for the atoms in thepresence of the electromagnetic radiation; and a data processor in datacommunication with the light detector and the data store. The dataprocessor measures an atomic spectrum for the atoms from the probinglight beam received from the light detector and analyzes the measuredatomic spectrum to extract spectral features. The data processor alsocompares the spectral features from the measured atomic spectrum tospectral features of the predetermined atomic spectra; and matches themeasured atomic spectrum to a given spectrum in the predetermined atomicspectra, thereby quantifying one of field strength or frequency of theunknown electromagnetic radiation field.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a diagram depicting an example measurement system;

FIGS. 2A and 2B are diagrams illustrating 3-level and 4-level atomicsystems, respectively;

FIG. 3 is a flowchart illustrating a technique for measuring theelectric field and/or frequency of electromagnetic radiation;

FIG. 4 is a block diagram of an experimental set up for the proposedelectric field measurement technique;

FIG. 5 is a graph showing a probe transmission as a function of Δ_(p)for the three-level EIT system;

${5S_{1/2}} - {5P_{\frac{3}{2}}} - {50D}$

FIG. 6 is a graph showing an EIT-signal as a function of Δ_(p) for theEIT system 5S_(1/2)-5P_(3/2)-28D_(5/2), and a signal when the 28D_(5/2)level is coupled to the 29P_(3/2) level by a 104.77 GHz RF field;

FIG. 7 is a graph showing experimental data for the measurement for Δfat 17.04 GHz;

FIG. 8 is a graph showing a comparison of experimental data to bothnumerical simulations and to far-field calculations for 15.59 GHz, 17.04GHz, and 104.77 GHz;

FIGS. 9A and 9B show weak-field measurement of 132.6495 GHz mm waves onthe 26D_(5/2)-27P_(3/2) one-photon transition versus √{square root over(power)}; and a strong-field measurement of 12.461 154 8 GHz microwaveson the 65D-66D two-photon transition versus power, respectively;

FIG. 10 is a graph showing the calculated |m_(j)|=½ (black circles) and3/2 (red circles) Floquet quasienergies and their relative excitationrates (circle area) from 5P_(3/2);

FIG. 11A shows an experimental spectra of the 65D-66D two-photontransition versus microwave power; and

FIG. 11B shows a composite Floquet map model of FIG. 11A.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

With reference to FIGS. 1 and 2, the basic concept for measuring theelectric field of electromagnetic radiation is presented. FIG. 1 depictsan example measurement system 10. The measurement system 10 is comprisedgenerally of a source of electromagnetic radiation 12, one or more lightsources 13, a light detector 14 and atoms of a known type contained in avacuum enclosure 15. In this example, a probing light beam is providedfrom a probing light source 13A and a coupling light beam is providedfrom a coupling light beam 13B. The atoms serve as the active medium forthe measurement probe. In this example, rubidium-85 (⁸⁵Rb) atoms arechosen as the active medium although other types of atoms fall withinthe scope of this disclosure, especially alkali atoms and those having asufficiently high vapor pressure at room temperature.

The measurement method is demonstrated using electromagneticallytransparency (EIT) in an atomic vapor or gas as an optical readout ofatomic structure that is representative of the electric field orfrequency of the electromagnetic radiation field of interest. Generally,an atom can be in different states with associated energies (levels).This is illustrated in FIG. 2 where the states of an atom are designatedby |i>, where “i” is a given state. FIG. 2A shows the three levels of anatom relevant to the optical EIT readout of atomic structure. Here, theprobe laser is resonant with the |1> to |2> transition (transition froma ground state to a first excited state) and a strong coupler laser isresonant with the |2> to |3> transition (transition from a first excitedstate to a Rydberg state). If the coupler laser is off, the probe lasergets scattered (absorbed and reemitted) by the atom. However, when thecoupler laser is on, there is an increased transmission of the probelaser due to quantum interference of excitation pathways when both theprobe is resonant with the |1> to |2> transition and the coupler isresonant with the |2> to |3> transition. This is the phenomenon of EIT.This provides a way to optically detect state |3> by measuring a changein the transmission of the probe through the atomic medium when, forexample, the coupler laser is scanned in frequency through |2> to |3>resonance. Thus, state |3> is detected as a narrow (EIT) transmissionpeak in the probe absorption. With the appropriate choice of opticalfields for the EIT, different atomic states (like |3> in this case) canbe optically interrogated.

When an electromagnetic radiation field interacts with an atom, theatomic structure, or its energy levels, can change. How the atomicstructure changes in this interaction depends on the nature (e.g.frequency and amplitude) of the electromagnetic radiation. FIG. 2Billustrates a special case of an interaction of the atom with anelectromagnetic radio-frequency (RF) field that is weak and resonantbetween two Rydberg levels |3> and |4>. In the case that the applied RFfield is weak, the interaction of the RF field with the atom leads to achange in the structure of the atomic energy levels (|3> and |4>) suchthat two dressed states are formed whose separation is proportional tothe electric field amplitude of the RF source. In this case, oneobserves a splitting of the original EIT peak into two peaks. Thissplitting is known as Autler-Townes (AT) splitting which is related tothe Rabi frequency Ω_(RF) of the |β

−|4

transition and applied field amplitude, and allows for a measurement ofthe E-field strength. In more general cases, for different RFfrequencies and amplitudes, the structure of the atomic energy levelschanges in different ways (not just a splitting) and more than just oneor two energy levels may change. These changes in the atomic structureare also detected optically using EIT, providing a measured spectrumwith different spectral features. The electric field or frequency of theelectromagnetic radiation is then determined by matching the measuredspectrum to a calculated spectrum in a set of predetermined spectra(previously calculated for the RF field frequency and amplitude range ofinterest) that uniquely corresponds to the electric field and frequencyof the RF field. The match is achieved by comparing the spectralfeatures (such as relative peak positions, peak heights, peak widths) ofthe measured spectrum to those of the calculated spectra in the set. Thematched predetermined spectrum, which was calculated for a specificelectric field amplitude and frequency, then provides the electric fieldamplitude and frequency of interest.

To measure the field strength (or amplitude) for different RF fieldfrequencies, different states |3

and |4

can be chosen. State |3

, with a state |4

to which the RF radiation field can couple, is selected by tuning thewavelength of the coupling laser. A large range of atomic transitionscan be selected, allowing measurements of RF fields over acorrespondingly wide selection of frequencies. In essence, the atoms actas a highly tunable, resonant, frequency selective RF detectors. This isa significant benefit of using Rydberg atoms as field probes. The widerange of states |3

selectable by the coupling laser and of states |4

available for RF measurement translates to the broadband nature of theprobe, which allows RF measurements ranging from 10 MHz to 500 GHz.

FIG. 3 depicts a proposed method for measuring the electric field (orfrequency) of electromagnetic radiation using the response of Rydbergatoms. To obtain a measurement of the EM radiation E-field strength itis critical that the atomic response be accurately modeled over thedesired range of EM radiation field frequencies and amplitudes. This isnecessary so that a comparison of the experimentally observed spectracan be made and information on the nature of the EM radiation field canbe obtained. The atomic response over a broad range of EM radiationfield frequencies and field strengths varies from linear regimes (wherethere is a linear response of the atomic level shifts to the incident EMradiation field) to highly non-linear regimes. Measurement of anarbitrary (unknown) EM radiation field therefore requires a completemodel of a broad range of atom-field interactions. As a starting point,a set of predetermined atomic spectra are provided at 31 for atoms of aknown type, where each of the predetermined atomic spectra models theresponse of the atoms to an electromagnetic radiation field. Techniquesfor calculating the predetermined atomic spectra are further describedbelow.

Electromagnetic radiation is propagated at 32 from a source towards theatoms residing in an active measurement region. In one embodiment, theatoms are in a gas state contained in a vacuum enclosure, such as avapor cell, which defines the active measurement region. Concurrently,light from one or more light sources is propagated at 33 through theatoms residing in the active measurement region, where the lightincludes at least one light field that is coupled to a Rydberg state ofthe atoms for measuring the atomic spectrum.

While the electromagnetic radiation is interacting with the atoms (orhas interacted with the atoms), an atomic spectrum is measured at 34using the light from the one or more light sources. In the exampleembodiment, the atomic spectrum is measured using electromagneticallyinduced transparency as is further described below. In otherembodiments, the atomic spectrum can be measured by (1) absorptionspectroscopy, wherein the spectrum is obtained by monitoring theabsorption of a light beam through the medium of atoms, (2) Rydberg-atomcounting via charged particle detectors or current measurement devices,wherein the Rydberg atoms are ionized and resulting charges are detectedby a measurement device. Other techniques for measuring an atomicspectrum also fall within the scope of this disclosure.

Next, the measured atomic spectrum is analyzed at 35 to extract spectralfeatures. In one example, the peaks in the measured spectrum arenumerically fit to Gaussians to extract features including relative peakpositions, peak heights, and peak widths. Other techniques forextracting spectral features from the measured atomic spectrum are alsocontemplated by this disclosure.

Extracted spectral features from the measured spectrum are then comparedat 36 to the spectral features of the predetermined atomic spectra. Forexample, the predetermined spectra shown as dots in FIGS. 9A and 9Bcontain information on the relative peak positions (signal positionsalong the vertical axes) and relative peak heights (size of blue dot)for two fixed electromagnetic radiation field frequencies (i.e.,132.6495 GHz in FIG. 9A and and 12.461 GHz in FIG. 9B) and differentelectric field amplitudes for each (top axes). Measured spectra arerepresented in gray scale in FIGS. 9A and 9B for different output powersof the electromagnetic radiation source. Note a single measured spectrumfor a specific output power is a vertical trace in the respective plot,with signal strength represented on a gray scale. The relative positionsof the signal peaks in each measured spectrum are matched at 37 to thecalculated spectrum with the closest relative peak positions. It isenvisioned that other spectral features in the measured spectrum, suchas peak height or peak width, can be used to determine the best matchwith a calculated spectrum. From the matched calculated spectrum, theelectric field (or frequency) of the electromagnetic radiation isquantified as indicated at 38. In this example, the field strength isread from the scale above the plot. It is to be understood that only therelevant steps of the methodology are discussed in relation to FIG. 3,but that other steps, including software-implemented instructions, maybe needed to control and manage the overall operation of the measurementsystem 10.

In the example embodiment, the measurement system 10 further includes adata processor (e.g. computer) and a data store (e.g., non-transitorycomputer memory). The data processor is in data communication with thelight detector and configured to receive a measure of the atomicspectrum from the light detector. The steps of analyzing the measuresatomic spectrum, comparing the spectral features from the measuredatomic spectrum to the spectral features of the predetermined atomicspectra, and matching the measured atomic spectrum to a given spectrumin the predetermined atomic spectra can be implemented by the dataprocessor. The predetermined atomic spectra are stored in the data storefor use by the processor.

Alternatively, within the limit of weak and resonant electromagneticradiation fields, the electric field can be obtained by a directmeasurement of the splitting between two peaks. The splitting isproportional to the electric field and the predetermined dipole momentof the transition between the resonantly coupled states. In this way,the electric field can be computed from the Rabi frequency as furtherdescribed below.

The method to determine the properties of an electromagnetic radiationfield from an optically measured atomic spectrum relies on havingaccurate models of the atomic response (spectra) over the fieldamplitude and frequency range of interest. Two models of the atomicresponse to electromagnetic radiation fields are described and validatedexperimentally. First, a perturbative model of the atomic response isimplemented that is valid for the special case of weak electromagneticradiation fields that are resonant or near-resonant with an atomictransition. Second, a complete non-perturbative model based on Floquettheory is described that is valid over the full range from weak tostrong electromagnetic fields that are either on resonance oroff-resonance with any atomic transition.

First, a simple model is implemented for use within the limit of weak EMfields resonant with an electric-dipole transition between the opticallyexcited Rydberg level |3

with another Rydberg level |4

. Here, the EM field splits the Rydberg-atom spectrum into two lines,known as the Autler-Townes effect. An example of such a splitting isseen in FIG. 6. In the limit of weak fields where higher-order effectscan be neglected, the splitting between the line pair, Δf, is identicalwith the Rabi frequency (Ω_(RF)=27πΔf) of the transition from level |3

to level |4

, and the electric field of the EM radiation is given by

$\begin{matrix}{{E_{RF}} = {{\frac{\hslash}{\wp \; {RF}}\Omega_{RF}} = {\frac{\hslash}{\wp \; {RF}}2{\pi\Delta}\; f}}} & (1)\end{matrix}$

where the unknown field, E_(RF), is proportional to the splitting,Planck's constant, and inversely proportional to the transition dipolemoment

_(RF), which quantifies the atomic response to the resonant field withinthis weak-field limit. The unknown field strength is calculated usingfirst principles, for example with the method given by T. F. Gallagherin “Rydberg Atoms”, Cambridge University Press, 1994. For embodimentsbased on rubidium atoms, in the calculation of the dipole moment

_(RF), one can use quantum defects described by W. Li et al in“Millimeter-wave spectroscopy of cold Rb Rydberg atoms in amagneto-optical trap: Quantum defects of the ns, np and nd series” andby M. Mack et al in “Measurement of absolute transition frequencies of85 Rb to nS and nD Rydberg states by means of electromagneticallyinduced transparency”.

When using room-temperature vapor cells and scanning the probe laserfrequency, differential Doppler shifts between the probe and couplingbeams alter the frequency separations between EIT peaks in the probetransmission spectrum. Splittings of 5P_(3/2) hyperfine states arescaled by 1−λ_(c)/λ_(p) while splittings of Rydberg states are scaled byλ_(c)/λ_(p). The latter factor is relevant to measurements of RF-inducedsplittings of EIT peaks and therefore is modified. With reference toFIG. 6, the frequency splitting of the EIT peaks in the probe spectrum,Δf, is measured and the E-field amplitude is then given by

$\begin{matrix}{{E_{RF}} = {2\pi \frac{\hslash}{\wp \; {RF}}\frac{\lambda_{p}}{\lambda_{c}}\Delta \; f}} & (2)\end{matrix}$

In the weak-field regime, predetermined spectra can be calculated withthis model for a fixed electromagnetic radiation frequency, associateddipole moment, and range of electric field amplitudes. A measuredspectrum, like the one shown in FIG. 6 (labeled “RF on”), is fit with adouble Gaussian fitting function to extract spectral features of themeasured spectrum including the splitting of the peaks. Here, thepredetermined spectra can then be searched for the predeterminedspectrum with a splitting that most closely matches the spitting of themeasured spectrum. The matched predetermined spectrum then provides theelectric field amplitude value corresponding to the measured splitting.Further, in the weak-field regime of the atom-field interaction, whereEquations (1) or (2) are valid, the electric field amplitude can beobtained from the splitting of the peaks of the measured spectrum. Inthe weak-field regime, the search and match process amounts todividing/multiplying with the predetermined dipole moment, Planck'sconstant, and any Doppler correction factors.

In an example embodiment, the measurement system relies upon onrubidium-85 (⁸⁵Rb) atoms as the active medium. As such, the probe lightis a 780 nm (“red”) laser and the |1

to |2

atomic resonance corresponds to the 5S_(1/2)-5P_(3/2) transition. Toensure that the |3

to |4

atomic resonance in ⁸⁵Rb is an RF transition, the |2

to |3

transition will correspond to a ˜480 nm (“blue”) laser. Inset of FIG. 7depicts a four-level atomic system. FIG. 7 shows the measured Δf as afunction of the square root of the RF signal generator (labeled as√{square root over (P)}_(SG)). It can be seen that the measured Δf islinear with respect to √{square root over (P)}_(SG) (noting |E|∝√{squareroot over (P)}_(SG)), as predicted for the case of weak fields. With themeasured splitting Δf, and the calculated electric-dipole matrixelement, the absolute field strength at the location for the lasers isobtained. Examples for three different RF fields, of differentfrequencies, are shown in FIG. 8 with a calculation of the expectedelectric field amplitude following Equation 2, validating this model ofthe atom-field interaction in this weak-field regime and itsimplementation in measuring the electric field amplitude.

An experimental setup used to demonstrate this optical measurementapproach is shown in FIG. 4. The experimental setup included a vaporcell 41, a horn antenna 42 (and a waveguide antenna is used for thehigher frequency measurements), a probe laser 43, and a coupling laser44, a lock-in amplifier 45, and a photo diode detector 46. In thisexample, the vapor cell is a glass cylinder with a length 75 mm and adiameter 25 mm containing (⁸⁵Rb) atoms. The levels |1

, |2

, |3

, and |4

correspond, respectively, to the ⁸⁵Rb 5S_(1/2) ground state, 5P_(3/2)excited state, and two Rydberg states. The probe laser is a 780 nm laserwhich is scanned across the 5S_(1/2)-5P_(3/2) transition. The probe beamis focused to a full-width at half-maximum (FWHM) of 80 μm, with a powerof order 100 nW to keep the intensity below the saturation intensity ofthe transition. FIG. 5 shows a typical transmission signal as a functionof relative probe detuning Δ_(p). The global shape of the curve is theDoppler absorption spectrum of ⁸⁵Rb at room temperature. To produce anEIT signal, a counterpropagating coupling laser (wavelength λ_(c)≈480nm, “blue”) is applied with a power of 22 mW, focused to a FWHM of 100μm. As an example, tuning the coupling laser near the 5P_(3/2)-50D_(5/2)Rydberg transition results in distinct EIT transmission peaks as seen inFIG. 5. The strongest peak at Δ_(p)=0 is labeled as “EIT signal”.

In order to improve the signal-to-noise ratio, heterodyne detection canbe used. The blue laser amplitude is modulated with a 30 kHz square waveand any resulting modulation of the probe transmission is detected witha lock-in amplifier. This removes the Doppler background and isolatesthe EIT signal as shown in the black curve of FIG. 6 (labeled as “RFoff”). Here, the coupling laser is tuned near the 5P_(3/2)-28D_(5/2)transition (“blue” with Δ_(c)≈482.63 nm). Application of a RF field at104.77 GHz to couple states 28D_(5/2) and 29P_(3/2) splits the EIT peakas shown in the gray curve (labeled as “RF on”). Using this heterodynedetection technique in the experimental data presented below results inmeasured EIT-signals with improved signal-to-noise ratio.

In weak RF-fields, the Rydberg levels are dynamically (ac) Stark-shiftedand, in the case of a near- or on-resonant drive of a Rydbergtransition, exhibit Autler-Townes splittings. For single-photontransitions in the weak-field limit, the RF E-field strength is directlyproportional to the Autler-Townes splitting of the Rydberg EIT line,which is given by the Rabi frequency Ω_(RF)=

_(RF)·E_(RF)/, where here again

_(RF) is the Rydberg transition dipole moment and E_(RF) is the RFradiation E-field vector.

FIG. 9A shows, as another example, experimental spectra for theon-resonant one-photon 26D_(5/2)-27P_(3/2) mm-wave (132.6495 GHz)transition as a function of the square root of mm-wave power. This issimilar to the previous example but for a higher frequencyelectromagnetic radiation field. Here, the EIT coupling-laser frequencyis resonant with the mm-wave-free ⁸⁵Rb 5P_(3/2)(F′=4)−26D_(5/2)transition, where F′ denotes the intermediate-state hyperfine component.As expected in weak RF fields, Ω_(RF) is a linear function of the squareroot of power (which is proportional to E_(RF)), making the splitting anexcellent marker for an electric field value associated with thatsplitting. The faint level pairs centered at about −70 and −110 MHzcorrespond to spectra associated with the intermediate 5P_(3/2)(F′=2, 3)hyperfine components. From the measured spitting, using Ω=

_(RF,z)E_(z)/ and the predetermined values of the dipole moments for az-polarized field d_(z) (405ea₀ for magnetic quantum number m_(j)=½ andthis transition), the E_(RF) fields obtained from the EIT spectra are inexcellent agreement with the predetermined spectra calculated followingEquation 2 (blue dots in FIG. 9). It is found that the maximum fieldamplitude measured in the experiment shown in FIG. 9A is 16 V/m, about0.02% of the microwave-ionization field of these atoms and well withinthe weak-field limit.

In FIG. 9A, the predetermined spectra (shown in blue) contains theinformation on the relative peak positions for 132.6495 GHz mm-wavesover a range of electric field amplitudes (top axis). Measured spectraare represented in gray scale in FIGS. 9A and 9B for different outputpowers of the electromagnetic radiation source. Note a single measuredspectrum for a specific output power is a vertical trace in therespective plot, with signal strength represented on a gray scale. Themeasured spectra are plotted as a function of the square root of theoutput power, which is proportional to the electric field in this linearregime as described previously. Here, both the frequency axis andelectric-field/root-power axis for the measured and calculated spectrahave the same, fixed relative step sizes.

To search the calculated spectra for a match with the measured spectra,the calculated spectra are overlaid on the measured spectra. Thecalculated spectra are first shifted vertically until the symmetrypoints between the measured and calculated splittings are equal. Thecalculated spectra are then shifted horizontally until the splittings ofthe measured and calculated spectra are equal. A match is obtainedbetween the measured and calculated spectra under the criterion that thesplitting of a calculated spectrum is equal to the measured spectrumwithin a fraction of the linewidth of the measured spectrum. Once thematch is obtained, the measured spectrum is now linked to the electricfield associated with the matched predetermined spectrum, therebyquantifying the electromagnetic radiation electric field for thatmeasured spectrum. In FIG. 9A, all of the measured spectra were matchedsimultaneously. However, it is envisioned that individual or a selectnumber of measured spectra can be matched following the same approach.

In an alternative approach, a more general model is set forth todetermine the atomic response to electromagnetic radiation fields. Thismore robust model can be used in the strong field regime, but it alsoapplies to the weak field regime as well, and for on-resonance andoff-resonance fields. To illustrate the atomic response in strongfields, measured and calculated atomic spectra for Rydberg atoms thathave been strongly driven at the zero-field 65D_(5/2)-66D_(5/2)two-photon resonance frequency (12.461 154 8 GHz) have been studied.This two-photon Rydberg transition is chosen to accommodate high-powermicrowaves in the K_(u) band. FIG. 9B shows experimental EIT spectracentered on the 65D level for injected microwave powers ranging from 13to 24 dBm in steps of 1 dBm. The 12 data sets are plotted as a functionof power. For this high-power measurement, the microwave-induced shiftsare in the range of several hundred MHz, i.e., about a factor of 10larger than the shifts of the low-power measurement in FIG. 9A. Asdetailed below, the maximum field reached in FIG. 9B is 230 V/m, about20% of the microwave-field-ionization limit for that case. It is notedthat the field-free fine structure of the 65D and 66D states(approximately 40 MHz) is broken up in the strong-field regime becauseit is small compared to the microwave-induced shifts. This aspectheralds the greater complexity of high-power Rydberg EIT spectracompared to low-power spectra, as evidenced by the comparison of FIG. 9Awith FIG. 9B.

At the lowest microwave power in FIG. 9B, the microwave interactionbroadens the 65D EIT resonance to a FWHM width of 2π×50±1 MHz, which isa factor of 2 larger than that of the microwave-free EIT resonance(not-shown). For increasing microwave power, the EIT signal splits intomultiple distinguishable spectral lines. For two-photon transitions, thetwo-photon Rabi frequency Ω_(RF)˜E_(RF) ². Hence, the lines are expectedto shift linearly as a function of RF power. Most levels in FIG. 9Bexhibit linear shifts up to microwave powers of approximately 70 mW.

In strong fields, higher-order couplings lead to a redistribution ofoscillator strength between many field-perturbed Rydberg states,resulting in smaller signal strengths compared to those in weak fields.This is reflected in FIG. 9B, where one observes a rapid initialdecrease in the signal strength; over the first 30-mW increase inmicrowave power, the peak signal strengths of the individual spectrallines reduce by more than an order of magnitude. As the microwave poweris increased further, the shifts of the spectral lines become nonlinearin power, reflecting substantial state mixing and higher-ordercouplings. The transition from linear to nonlinear behavior occursgradually as a function of power, also, the details of this transitionvaries from level to level. As seen by close inspection of FIG. 9B, evenat the lowest powers most levels exhibit some degree of nonlinearity. Aquantitative model of the complex level structure in the strong-fieldregime is described below.

Inhomogeneous fields within the measurement volume contribute to thebackground and additional spectral lines, which are observed in FIG. 9B.The field inhomogeneity is attributed in part to the presence of thedielectric cell, and to the fact that the measurement is done in thenear-field of the microwave horn. The effects of the field inhomogeneityare also discussed in detail in sections below.

In strong fields, where typical Rabi frequencies approach or exceedatomic transition frequencies, high-order couplings become significantand perturbative approaches are no longer valid. To model thestrong-field experimental spectra, a (non-perturbative) Floquet methodis adequate. Following the Floquet theorem, the solutions toSchrödinger's equation for a time-periodic Hamiltonian Ĥ(t)=Ĥ(t+T),where T is the period of the rf field, are of the form

Ψ_(v)(t)=e ^(−iW) ^(v) ^(t/)  (13)

Here, Ψ_(v)(t)=Ψ_(w)(t+T) are the periodic Floquet modes and W_(v) theirquasienergies, with an arbitrary model label v. For the atom-fieldinteraction strength of interest here, the Floquet modes can berepresented using standard basis states |n,l,j,m_(j)

=|k

, i.e.,

Ψ_(v)(t)=e ^(iW) ^(v) ^(t/)Σ_(k) C _(v,k)(t)|k)  (14)

with time-periodic (complex) coefficient functions that satisfyC_(v,k)(t)=C_(v,k)(t+T). The Floquet energies W_(v) and statesΨ_(v)(t=0) are determined by finding the eigenvalues and vectors of thetime-evolution operator Û(t,T+t). The coefficient functions C_(v,k)(t)are then obtained by integrating Ψ_(v)(t) over one period of the RFfield, t∈[0,T].

In the laser excitation of Floquet states from the intermediate 5P_(3/2)state, multiphoton processes are important because the atom may emit orabsorb a number of microwave photons together with an optical photon. Tocompute excitation line strengths, the above functions C_(v,k)(t) areFourier-expanded:

$\begin{matrix}{{{{\Psi_{v}(t)} = {{^{{- }\; W_{v}{t/\hslash}}\Sigma_{k}\Sigma_{N = {- \infty}}^{\infty}{\overset{\sim}{C}}_{v,k,N}^{{- }\; {Nw}_{rf}t}}k}}\rangle},{{\overset{\sim}{C}}_{v,k,N} = {\frac{1}{T}{\int_{0}^{T}{{C_{v,k}(t)}^{\; {Nw}_{rf}t}{{t}.}}}}}} & (15)\end{matrix}$

The integer N is interpreted as a number of microwave photons withfrequency ω_(rf) associated with the bare atomic state. The laserfrequencies ω_(L), where Floquet levels are resonantly excited from the5P_(3/2) level, and the corresponding line strengths S_(v,N) are thengiven by:

w _(L) =W _(v) +Nw _(rf),

S _(v,N)=(eE _(L)/)²|Σ_(k) {tilde over (C)} _(v,k,N) {circumflex over(∈)}·

k|{circumflex over (r)}|5P _(3/2) ,m _(j)

|²  (16)

where E_(L) is the amplitude of the laser E field, {circumflex over (∈)}is the laser-field polarization vector, and

k|{circumflex over (r)}|5P_(3/2),m_(j)

are the electric-dipole matrix elements of the basis states with|5P_(3/2),m_(j)

. Each Floquet level W_(v) leads to multiple resonances, which areassociated with the microwave photon number N. Because of parity, in theabsence of additional static fields, a Floquet level W_(v) may generateresonances for either even N or odd N but not both.

In FIG. 10, calculated Floquet energies and excitation rates S_(v,N) areshown in the vicinity of the 65D Rydberg level for a microwave frequencyof 12.461 154 8 GHz and field strengths ranging from 0 to 350 V/m. Thefield is displayed on a quadratic scale to show the dependence of theatomic-level shifts on power and for direct comparison with FIG. 9B.Over the limited frequency range displayed in FIG. 10, it is always N=0.Further inspection of the calculated Floquet energies and excitationrates, not presented in detail here, shows that for fields aboveapproximately 150 V/m several Floquet levels W_(v) visible in FIG. 10have copies with high excitation rates for even values of N betweenabout −8 and +8.

The Floquet modes in strong fields exhibit nontrivial wave-packetmotion, and their optical excitation rates have to be calculatedaccording to Eq. (16). Low-field approximations along the lines ofEquations (1) and (2) are not valid. It would, for instance, beincorrect to associate the excitation rates of the Floquet modes inFIGS. 10 and 11, with the 65D probabilities the modes carry. Toqualitatively explain this, it is noted that in weak fields thedressed-state coefficients and dipole moments have fixed amplitudes andphases relative to the field (in the field picture and using therotating-wave approximation). In strong fields, wave-functioncoefficients and dipole moments vary significantly throughout themicrowave-field cycle. Specifically, the Floquet modes are time-periodicwave packets that are synchronized with the driving rf field.

In FIG. 9B, one finds an excellent overall agreement between dominantfeatures in the experimental and theoretical Floquet maps. Additionalfeatures evident in the experimental map are due to E-fieldinhomogeneities, which are discussed below. Both experimental andtheoretical maps exhibit several arched lines at positive frequenciesand, at negative frequencies, several lines that shift approximatelylinearly in power. The dominant downshifting line suddenly terminates atclose to −400 MHz. It is evident from FIG. 10 that the suddentermination of the downshifting lines is due to a wide Floquet avoidedcrossing. Avoided crossings in Floquet maps, such as those in FIG. 10,provide convenient markers for spectroscopic determination of the rf Efield on an absolute scale. In the present case, the prominent avoidedcrossing at 165 V/m (see label 2 in FIG. 10) has several matchinglocations in the experimental spectrum shown in FIG. 19B. These are seenmore clearly in FIG. 11A, which shows the experimental data on a dBmscale. The appearance of multiple copies of the calculated avoidedcrossing in the experiment points to the fact that the microwave fieldwithin the cell had multiple dominant domains, each of which producedits own rendering of the avoided crossing. The rendering at the lowestinjected microwave power corresponds to the E field domain with thehighest field at a given injected power [calculation and E field axisshown in FIG. 9B]. In FIG. 9B, the avoided crossing is observed first at130 mW, at which point the domain that has the highest field reaches 165V/m. It follows that at a microwave power of 250 mW a maximum RF E fieldof 230±14 V/m is reached. The uncertainty of this value is given by halfthe experimental power step size (±0.5 dB, corresponding to ±6% infield).

Using Floquet models, a method for measuring the electric field (orfrequency) of electromagnetic radiation proceeds as follows. First, theFloquet model is used to calculate predetermined spectra for theelectromagnetic radiation over the electric field and frequency rangesof interest. A measured spectrum, like any of the ones shown in FIG. 9B,is analyzed to extract spectral features of the measured spectrumincluding relative peak positions, peak heights, and peak widths. Thepredetermined spectra can then be searched for the predeterminedspectrum with the features that most closely match those of the measuredspectrum. For example, due to the complex non-linear features of thespectra, the search entails first shifting the overlaid calculatedspectra vertically over the measured spectra so that the zero-field (RFoff) calculated spectrum and measured spectrum are both at the samefrequency (zero in the plot). The calculated spectrum is then shiftedhorizontally until the positions of the various peaks in the measuredand calculated spectra overlap. In this example, the predeterminedspectra are calculated at a fixed frequency over a range of fieldstrengths. By matching the measured spectrum to the calculatedpredetermined spectra, one can quantify the electric field of theelectromagnetic radiation field associated with each of the measuredspectra. The quality of the overlap and match is determined by how wellthe peak positions of the calculated spectra fall within the measuredpeak positions. Different criteria for this match will result indifferent uncertainties in the final field value. In one example, thecriterion is that the calculated peak should fall within the full widthhalf maximum of the measured signal. In this example, the uncertainty isdominated by the experimental step size, not the matching criteria.There are also considerations of unaccounted for peaks in the spectradue to, for example, field inhomogeneities, which can also be modelled.It is emphasized that the matching process is necessary to obtain theelectric field for atomic spectra in a medium to strong field regimewhere simple models like Equations 1 and 2 do not apply. The proposedapproach provides a more general way to measure such properties of theelectromagnetic radiation field of interest. As before, the describedprocess is for matching a series of measured spectra at the same time. Asingle measured spectrum can be matched to a predetermined spectrumfollowing a similar procedure.

Furthermore, a measurement of the frequency of the electromagneticradiation field can be achieved by calculating predetermined spectra ata fixed electric field value for a range of electromagnetic fieldfrequencies. This results in predetermined spectra that are linked tothe frequency for the radiation field that can then be search andmatched with a measured atomic spectra as described above to obtain avalue for the electromagnetic field frequency. A simultaneousmeasurement of electric field and frequency can also be achieved bycalculating a series of predetermined calculated spectral maps like theones shown in FIG. 9 for a range of frequencies. This would result in alarger set of predetermined spectra that can be searched in bothfrequency and field amplitude for a match with the measured spectrum.

In the following sections, an explanation is provided of the analysisused to model the experimental spectra plotted in FIGS. 9B and 11A. Thespectra contain information on the continuous distribution of themicrowave E field strength along the length of the EIT probe andcoupling beams passing through the spectroscopic cell. The microwaveboundary conditions are symmetric about the xy plane, with the incidentmicrowave E field being z polarized and the optical beams propagatingalong the x axis. Since the microwave field is primarily z polarizedalong the optical beams, it drives Δm_(j)=0 Rydberg transitions with them_(j)=±½ and m_(j)=±3/2 manifolds of states. The field distribution is aresult of superpositions of reflections from the cell walls. Also, thecell is placed within the near field of the source, leading toadditional variability of the microwave field along the probe beam.Hence, one may picture the RF field as a speckle pattern, akin tospeckle patterns seen for general, nonideal coherent fields. Here, oneshould expect the number of speckles to be on the order of the celllength divided by the wavelength, which in this case is 3. Also, sincethere are no structures within the cell that are very close to theoptical probe beam path, one would not expect any sharp spatial featuresin the microwave E field (features which might otherwise arise fromsharp metallic or dielectric edges and the like). Therefore, for eachtheoretical line in FIG. 10, the measured EIT spectra are expected toexhibit a small number of spectral features that correspond to the localmaxima and minima of the microwave field along the length of the probebeam.

Based on the observation of five downward spectral lines [labeled inFIG. 11A] and the fact that the calculated spectrum has only onecorresponding downward line (feature 1 in FIG. 10), the spectrum ismodeled considering populations of atoms located within a set of fivedominant microwave E field regions. In the model, the probabilitydistribution for intensity on a decibel scale is given by

P _(dBi)(s)=Σ_(|m) _(j) _(|=1/2) ^(3/2)Σ_(k=1) ⁵ w _(m) _(j) (|m _(j)|)w_(k)(k)P _(dBi0)(s+Δs _(k)).  (17)

Here, k is an index for the five microwave field domains, w_(k)(k) isthe probability that an atom contributing to the signal resides withindomain k, w_(mj) (|m_(j)|) is the probability that an atom contributingto the signal has a magnetic quantum number |m_(j)|, and P_(dBi0) is aGaussian point-spread function that accounts for inhomogeneous spectralbroadening within the five domains. The values Δs_(k) indicate by whatamount (in decibels) the central microwave intensity within the kthmicrowave field region is shifted relative to the intensity in thehighest-intensity (k=1) domain. For P_(dBi0), it is assumed a Gaussianthat is the same for all k. The fit parameters in the model are Δs_(k),w_(m) _(j) ,w_(k), and the standard deviation d_(dBi0) for P_(dBi0). Onecan account for the optical EIT line broadening and laser line drifts bya Gaussian spread function in frequency, P_(v)(v), which has a standarddeviation σ_(y). From the theoretical spectrum S_(T)(s,v), the modelspectrum S_(E)(s₀s_(v)) is then given by the convolution

S _(E)(s ₀ ,v ₀)=∫S _(T)(s ₀ −s′,v ₀ +v′)P _(v)(v′)P_(dBi)(s′)dv′ds′,  (18)

where the intensities in the arguments of S_(E) and S_(T) are measuredin dBi, defined as 10 log₁₀ [I/(W/m²)], where I is the RF fieldintensity.

FIG. 11B shows the model spectrum S_(E)(s₀,v₀) in dBi for the data inFIG. 11A. In the model spectrum, the field domains have empiricallyfitted intensity shifts of λ_(S) _(k) =0.0, −2.0, −4.17, −6.0, and −8.0db for k=1, . . . , 5. The corresponding fitted weighting factors arew_(k)=0.39, 0.21, 0.27, 0.09, and 0.04. The intensity shifts aresignificant to better than about 0.5 dB, while the weighting factors aresignificant to better than about ±0.04. The weighting factors w_(m) _(j)for the |m_(j)|=½ and 3/2 states are 0.7 and 0.3, respectively(significance level better than about 0.1). The larger |m_(j)|=½ weightlikely results from optical pumping by the EIT probe field. Furthermore,σ_(dBi)=1 dBi and σ_(v)=30 MHz.

A comparison of the measured spectrum and the model spectrum in FIG. 11shows that a strong-field Floquet analysis of the atomic physics ofRydberg atoms in microwave fields, combined with a straightforwardempirical model of the microwave intensity distribution and theweighting in the sample, leads to remarkably good agreement betweenspectra with rather complex features. Utilizing a combination ofresonant, strong electric-dipole transitions as in FIG. 9A andhigher-order transitions such as the two-photon transition in FIG. 9B,it is possible to observe level shifts in Rydberg EIT spectra over awide dynamic range of the applied RF intensity.

The models of the atom-field interactions described herein (Autler-Townsmodel and Floquet model) depend only on invariable atomic parameterssuch as quantum defects and dipole moments, and fundamental constants,such as Planck's constant. The method to determine the electric field orfrequency of an unknown electromagnetic radiation field using thesemodels therefore provides a measurement that is directly SI traceable inwhich the uncertainty of each step in the computation iswell-characterized and documented. The uncertainty of the dipole momentcalculation is considered to be less than 0.1%, so the overalltraceability to SI units has a correspondingly small uncertainty,compared to previous E-field measurement techniques.

Since the spectroscopic response is well described by the Floquet theorylaid out above, predetermined spectra for a chosen RF frequency andfield amplitude range, and measured spectrum can be used together asdescribed above to quantify the RF E field causing the observed spectralfeatures in a calibration-free manner. Specifically, there are noantenna systems and readout instruments that need to be calibrated totranslate a reading into a field, because spectral features such as lineshifts and avoided crossings follow from the invariable nature of theunderlying atomic physics. The field measurement precision is given byhow well the spectral features are resolved. For instance, in thepresent disclosure, the avoided crossing pointed out by the arrows inFIGS. 10 and 11 can be resolved to within ±0.5 dBi uncertainty,corresponding to an absolute field uncertainty of ±0.6%.

While in the experimental examples shown here a cell on the order of 25mm to 75 mm is used, the vapor cell can be made smaller and hence allowa compact probe (or sensor head).

Regardless of the size of the vapor cell, this technique allows forsub-wavelength imaging of an RF field over a large frequency range. Thishas been demonstrated where field distributions inside a glass cell wereimaged at both 17.04 GHz and 104.77 GHz. The unique feature of thisimaging approach is that the spatial resolution is not governed by thesize of the vapor cell that holds the atoms. The RF field will onlyinteract with the atoms that are exposed to the two laser beams. Assuch, the spatial resolution of this approach is based on beam widths ofthe two lasers used in this experiment, which can be in principle on theorder of the diffraction limit, i.e., 10's of mircometers. Theapplications of such a small spatial imaging capability are numerous.For example, the sensing volume could be scanned over aprinted-circuit-board (PCB) or a metasurface in order to map theirfields, as well as other applications where E-field measurements on asmall spatial resolution are desired.

The techniques described herein may be implemented in part by one ormore computer programs executed by one or more processors. The computerprograms include processor-executable instructions that are stored on anon-transitory tangible computer readable medium. The computer programsmay also include stored data. Non-limiting examples of thenon-transitory tangible computer readable medium are nonvolatile memory,magnetic storage, and optical storage.

Some portions of the above description present the techniques describedherein in terms of algorithms and symbolic representations of operationson information. These algorithmic descriptions and representations arethe means used by those skilled in the data processing arts to mosteffectively convey the substance of their work to others skilled in theart. These operations, while described functionally or logically, areunderstood to be implemented by computer programs. Furthermore, it hasalso proven convenient at times to refer to these arrangements ofoperations as modules or by functional names, without loss ofgenerality.

Unless specifically stated otherwise as apparent from the abovediscussion, it is appreciated that throughout the description,discussions utilizing terms such as “processing” or “computing” or“calculating” or “determining” or “displaying” or the like, refer to theaction and processes of a computer system, or similar electroniccomputing device, that manipulates and transforms data represented asphysical (electronic) quantities within the computer system memories orregisters or other such information storage, transmission or displaydevices.

Certain aspects of the described techniques include process steps andinstructions described herein in the form of an algorithm. It should benoted that the described process steps and instructions could beembodied in software, firmware or hardware, and when embodied insoftware, could be downloaded to reside on and be operated fromdifferent platforms used by real time network operating systems.

The present disclosure also relates to an apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general-purpose computerselectively activated or reconfigured by a computer program stored on acomputer readable medium that can be accessed by the computer. Such acomputer program may be stored in a tangible computer readable storagemedium, such as, but is not limited to, any type of disk includingfloppy disks, optical disks, CD-ROMs, magnetic-optical disks, read-onlymemories (ROMs), random access memories (RAMs), EPROMs, EEPROMs,magnetic or optical cards, application specific integrated circuits(ASICs), or any type of media suitable for storing electronicinstructions, and each coupled to a computer system bus. Furthermore,the computers referred to in the specification may include a singleprocessor or may be architectures employing multiple processor designsfor increased computing capability.

The algorithms and operations presented herein are not inherentlyrelated to any particular computer or other apparatus. Variousgeneral-purpose systems may also be used with programs in accordancewith the teachings herein, or it may prove convenient to construct morespecialized apparatuses to perform the required method steps. Therequired structure for a variety of these systems will be apparent tothose of skill in the art, along with equivalent variations. Inaddition, the present disclosure is not described with reference to anyparticular programming language. It is appreciated that a variety ofprogramming languages may be used to implement the teachings of thepresent disclosure as described herein.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A method for measuring the electric field ofelectromagnetic radiation using the spectroscopic responses of Rydbergatoms to the radiation to be measured, comprising: providingpredetermined atomic spectra for atoms of a known type; placing theatoms within the unknown electromagnetic radiation field to be measured,where the atoms are in a gaseous state and contained in a vacuumenclosure; propagating one or more light beams through the atoms, whereat least one light beam is coupled to a Rydberg state; measuring anatomic spectrum using the one or more light beams while the unknownelectromagnetic radiation is interacting with or has interacted with theatoms; analyzing the measured atomic spectrum to extract spectralfeatures; comparing the spectral features from the measured atomicspectrum to spectral features of the predetermined atomic spectra;matching the measured atomic spectrum to a given spectrum in thepredetermined atomic spectra; and quantifying at least one of fieldstrength or frequency of the unknown electromagnetic radiation fieldusing the given spectrum and the predetermined atomic spectra.
 2. Themethod of claim 1 wherein providing predetermined atomic spectra foratoms further comprises determining a model of atomic response inpresence of the electromagnetic radiation.
 3. The method of claim 1wherein providing predetermined atomic spectra for atoms furthercomprises calculating the predetermined atomic spectra at a fixedfrequency for a range of electric field values.
 4. The method of claim 1wherein providing predetermined atomic spectra for atoms furthercomprises calculating the predetermined atomic spectra at a fixedelectric field for a range of frequencies.
 5. The method of claim 1wherein providing predetermined atomic spectra for atoms furthercomprises calculating the predetermined atomic spectra using Floquettheory.
 6. The method of claim 1 wherein the atoms contained in thevacuum enclosure are maintained at a fixed temperature and density. 7.The method of claim 1 wherein measuring an atomic spectrum usingelectromagnetically induced transparency.
 8. The method of claim 5wherein measuring an atomic spectrum of the atoms further comprisespropagating a probing light beam through the atoms, where the probinglight beam has a frequency resonant with transition of the atoms from afirst quantum state to a second quantum state; propagating a couplinglight beam through the atoms simultaneously with the probing light beam,where the coupling light beam is overlapped spatially with the probinglight beam, frequency of the coupling light beam is scanned across arange in which atoms transition from the second quantum state to aRydberg state; and detecting the probing light beam passing though theatoms using a light detector.
 9. The method of claim 7 wherein measuringan atomic spectrum of the atoms further comprises propagating a probinglight beam through the atoms, where frequency of the probing light beamis scanned across a range in which atoms transition from a first quantumstate to a second quantum state; and propagating a coupling light beamthrough the atoms concurrently with the probing light beam, where thecoupling light beam is overlapped spatially with the probing light beam,frequency of the coupling light beam is resonant with transition of theatoms from the second quantum state to a Rydberg state; and detectingthe probing light beam passing though the atoms using a light detector.10. The method of claim 1 wherein the spectral features extracted fromthe measured atomic spectrum are defined as the frequency differencebetween two split peak pairs in the measured atomic spectrum.
 11. Themethod of claim 10 wherein comparing the spectral features from themeasured atomic spectrum further comprises overlaying the predeterminedatomic spectra onto the measured atomic spectrum and shifting thepredetermined atomic spectra such that the predetermined atomic spectraaligns with the measured atomic spectrum.
 12. The method of claim 11wherein quantifying field strength of the unknown electromagneticradiation field further comprises determining Rabi frequency from asplitting of a Rydberg line in the measured atomic spectrum, calculatingdipole moment of the relevant Rydberg transition, and computingmagnitude of field strength of the unknown electromagnetic radiationfield from the Rabi frequency and the dipole moment.
 13. The method ofclaim 1 wherein the spectral features extracted from the measured atomicspectrum are defined as one or more of peak heights, peak widths andrelative peak positions in a Floquet map.
 14. The method of claim 13wherein comparing the spectral features from the measured atomicspectrum further comprises overlaying the predetermined atomic spectraonto the measured atomic spectrum, shifting the predetermined atomicspectra in relation to the measured atomic spectrum so that the spectralfeatures are in agreement, thereby yielding the field strength orfrequency of the unknown electromagnetic field.
 15. The method of claim1 further comprises measuring the atomic spectrum without the necessityof metal or conductive material in the vacuum enclosure.
 16. A methodfor measuring the electric field of electromagnetic radiation using thespectroscopic responses of Rydberg atoms to the radiation to bemeasured, comprising: calculating predetermined atomic spectra for atomsof a known type using Floquet theory; propagating an unknownelectromagnetic radiation field towards the atoms, where the atoms arein a gas state and contained in a vacuum enclosure; propagating one ormore light beams through the atoms, where at least one light beam iscoupled to a Rydberg state of the atoms; measuring an atomic spectrumusing the one or more light beams while the unknown electromagneticradiation is interacting with or has interacted with the atoms;analyzing the measured atomic spectrum to extract spectral features;comparing the spectral features from the measured atomic spectrum tospectral features of the predetermined atomic spectra; and matching themeasured atomic spectrum to a given spectrum in the predetermined atomicspectra, thereby quantifying one of field strength or frequency of theunknown electromagnetic radiation field.
 17. The method of claim 16further comprises measuring an atomic spectrum using electromagneticallyinduced transparency.
 18. The method of claim 16 further comprisesmeasuring an atomic spectrum using electromagnetically inducedtransparency.
 19. The method of claim 16 further comprises analyzing themeasured atomic spectrum to extract peak positions and comparing thepeak positions from the measured atomic spectrum to peak positions ofthe predetermined atomic spectra by overlaying the predetermined atomicspectra onto the measured atomic spectrum and shifting the predeterminedatomic spectra in relation to the measured atomic spectrum until thepeak positions in the predetermined atomic spectra fall within fullwidth half maximum of the peak positions in the measured atomicspectrum.
 20. A system for measuring the electric field ofelectromagnetic radiation using spectroscopic responses of Rydbergatoms, comprising: a vapor cell containing atoms of a known type; asource of electromagnetic radiation arranged to emit electromagneticradiation towards the vapor cell; a probing light source configured topropagate a probing light beam through the vapor cell, where frequencyof the probing light beam is scanned across a range in which the atomstransition from a first quantum state to a second quantum state; acoupling light source configured to propagate a coupling light beamthrough the vapor cell concurrently with the probing light beam, wherethe coupling light beam is counterpropagating to and overlappedspatially with the probing light beam, and frequency of the couplinglight beam is resonant with transition of the atoms from the secondquantum state to a Rydberg state; a light detector configured to receivethe probing light beam after passing through the vapor cell; a datastore that stores predetermined atomic spectra for the atoms in thepresence of the electromagnetic radiation; and a data processor in datacommunication with the light detector and the data store, and operatesto measure an atomic spectrum for the atoms from the probing light beamreceived from the light detector and analyze the measured atomicspectrum to extract spectral features, wherein the data processorcompares the spectral features from the measured atomic spectrum tospectral features of the predetermined atomic spectra; and matches themeasured atomic spectrum to a given spectrum in the predetermined atomicspectra, thereby quantifying one of field strength or frequency of theunknown electromagnetic radiation field.